Hollow cathode device with improved spectral light output and stability

ABSTRACT

A hollow cathode lamp of the type used as a source of spectral radiation characteristic of the materials within the hollow cathode. More particularly, the cathode is comprised of a body of material of which a characteristic radiation is desired having hollow within a front surface and an electrical conductive sleeve member surrounding said cathode body and extending beyond the front surface of the cathode body and electrically interconnected with the cathode lead-in.

1451 Apr.3, 1973 United States Patent Yamasaki PATENTEUAPM 1973 SHEU l UF 2 j l 1/IS POWER SUPPLY FIG. 3

III

FIG.

PATENTEUAPRs 1975 RELATIVE SPECTRAL INTENSITY SIIEEI 2 F 2 lo l5 2o 2'5 OPERATING CURRENT 1N MA.

FIG. 4

CURRENT- FIG. 5

HOLLOW CATHODE DEVICE WITH IMPROVED SPECTRAL LIGHT OUTPUT AND STABILITY BACKGROUND OF THE INVENTION This invention relates to a source of spectral radiation of the hollow cathode type which is adapted for emitting radiation having defined spectral lines characteristic of the material within the hollow cathode. One particular application of these light sources is in spectroscopic investigations. There have been a considerable number of hollow cathode devices provided which both emit radiation of a single element or emit radiation of several elements from a common cathode. Several elements such as arsenic, tellurium, selenium,

antimony, sulfur and phosphorous have a high cathode fall, a tendency to sublime and/or a relatively high vapor pressure. With these materials, spurious spectral noise may be generated because of arcing between the cathode and adjacent insulating shield junctures where high electrical field effects become pronounced. A typical configuration is shown in U.S. Pat. No. 3,264,591. It is also found with these materials that small cathode temperature changes resulting from small cathode current changes and cathode fall fluctuations can introduce noise and drift into the desired spectral output from the -hollow cathode.

SUMMARY OF THE INVENTION A new and improved hollow cathode device is provided wherein an electrical conductive sleeve member surrounds the hollow cathode and extends beyond the front surface of the hollow cathode and serves as a current regulator to render the spectral output of the hollow cathode less sensitive to changes in the operational current. Also by providing a relatively good electrical conductor adjacent the insulating support of the cathode, substantial elimination of arcing at the juncture of the cathode with the insulating shield is obtained. v

DESCRIPTION OF THE DRAWING ters in the spectral emission from a hollow cathode with respect to cathode current; and

FIG. 5 is a set of curves illustrating cathode fall potential versus current for different devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FlG. 1, there is shown a hollow cathode spectral device 10 in accordance wit the present invention. The device 10 includes an envelope 11, made of a suitable insulating material such as glass and having an enlarged tubular portion 12 and a smaller tubular portion 14 interconnected by a transition portion 13. The tubular portion 14 is sealed off at one end by a window 16 which is of a suitable material such as quartz, borosilicate, magnesium fluoride or lithium fluoride transmissive to radiation wavelengths produced by the device. The tubular portion 12 is sealed off by a button stem 18 with a tipped off exhaust tubulation 20. The

envelope l 1 is filled with a suitable gas such as argon or neon at a pressure of about l to 2O millimeters of mercury.

A cathode assembly 22 is provided within the envelope 11 and is supported by a lead-in member 26. The cathode assembly 22 comprises a cylindrical member 17 having a centrally disposed bore 18 extending from the front surface of the cylindrical member 17 facing the window 16 into the member 17 for a distance of about 30 mm and having a diameter of 9 mm. The cylindrical member 17 may be of any suitable electrical conductive material such as aluminum, graphite, iron, copper, brass or silver. Positioned within the bore 18 is the active cathode element or body 19 which is also a cylindrical member. The front surface 20 of the cathode body 19 facing the window 16 is disposed within the bore 18 at a distance of about 4 and l2 mm from the front surface of the member 17. The front surface 20 also has a centrally disposed bore 21 of a diameter of about 4 mm and extending to a depth of I about 7 mm into the cathode body 19. The bore 21 is the active region of the cathode assembly 22. The cathode element 19 is comprised of material such as arsenic, tellurium, selenium, antimony, sulfur or phosphorous. These materials may be alloyed or combined with easily sputtered elements such as silver, iron, copper, indium, gold, nickel and palladium. A specific example is a mixture of 50 percent silver and 50 percent arsenic by weight. Other suitable mixtures by weight are 8l percent silver and 19 percent arsenic; and 78 percent silver and 19 percent arsenic and 3 percent bismuth. The choice and percentage of materials is large. The cathode 19 is normally formed by casting or powder metallurgical techniques and after its form ation is inserted into the sleeve 17 and secured therein by suitable means. For example, a metal foil such as aluminum may be wrapped around the cathode 19 and then the cathode forced into lthe sleeve 17. This structure works particularly well for arsenic, tellurium and selenium.

FIG. 2 illustrates a possible modification of the cathode assembly 22 shown in FIG. l. In this structure, rather than utilize an electrically conductive sleeve, an insulating sleeve 23 is utilized of a suitable material such as ceramic and having a wall thickness of about l to 2 mm. An electrically conductive lining 24 is provided on the inner surface of this sleeve 23. The-lining 24 may be of a foil or coating of electrical conductive material such as aluminum, aquadag (graphite), gold, iron, copper, brass or silver. The conductive lining 24 may also be applied by painting or vacuum deposition onto the sleeve 23.

An anode element 28, which is illustrated as a ring member is positioned in close proximity to the cathode assembly 22 near the front surface of the cathode assembly 22. The anode element 28 is made of a suitable electrical conductive material such as tantalum or nickel or any metal with a relatively high melting temperature and is supported within the envelope 11 by means of two support rods 30. At least one of the support rods 30 is of a suitable electrical conductive material such as nickel. The rods 30 are secured to the anode 28 and are supported and secured through the button stem 18. A potential may be applied to one of the support rods 30 for application of a potential to the anode 28. A DC potential source 3l of about 500 volts is connected to the lead 30 and the lead 26 through a suitable resistor 33. The resistance of the resistor 33 may be about 20,000 ohms. This is illustrated in FIG. 3.

In order to limit the path of an electrical discharge between the cathode assembly 22 and the anode element 28, shielding means is provided which comprises two shielding disks 38 and 44 which may be of insulating material and disposed in spaced parallel relationship between the cathode assembly 22 and the anode element 28. More specifically, the shielding disk 38 has an aperture 40 therein which is disposed concentrically about the hollow bore 21 of the cathode element 19. Further, the shielding disk 38 may be disposed so as to be in close proximity to or abut with the front edge of the sleeve 17 and extends therefrom to the inner surface of the envelope l1. The shielding member 38 tends to confine the electrical discharge between the anode 28 and cathode assembly 22 to within the sleeve- 17 and primarily to the bore 21. The disk 44 has an aperture 4l into which the cathode assembly 22 is positioned and the disk 44 extends from the outer surface of the cathode 22 to the inner surface of the envelope 1l. This disk 44 may be of an insulating material. In addition, the shielding means includes a pair of insulating sleeves 48 disposed about the support rods 30 and extending between the button stem 18 and the insulating disk 44. Insulating rings 47 are disposed about the support rods 30 between the disks 44 and 38 and insulating rings 46 are disposed about the rods 30 between the disk 38 and the anode element 28. A more detailed description of this structure may be found in U.S. Pat. No. 3,264,511.

In the operation of the device, a suitable potential from the source 31 is applied between the cathode assembly 22 and the anode 28 so that an electrical discharge occurs between the cathode element 19 and the anode element 28 through the gaseous medium within the envelope 1l. This electric discharge creates positive ions of gas which in turn bombard the inner surface of the bore 2l of the cathode element 19 thereby sputtering atoms of the material such as arsenic and silver off of the walls of the bore 21. These sputtered atoms are excited by collision with metastable gas fill atoms present in the discharge. These excited atoms emit light of wavelengths characteristic of the two materials namely arsenic and silver. This light is emitted through the window 16.

Referring now to FIG. 3, a current Ic flows betwee the anode 28 and the cathode 19 and determines the spectral output of the hollow cathode device. The current lc is only a fraction of the total current from the power supply 31. Any fluctuation or change in the current Ic will cause a change in the spectral output of a hollow cathode. It will also produce a change in the sleeve current Is between the anode 28 and the sleeve 17. The current Is flowing through the sleeve 17 will tend to reduce the effect that which occurs in the cathode circuit 19. -For example, in an arsenic device, as the arsenic cathode becomes heated from operation,

the arsenic becomes more volatile. This causes the' cathode fall at the hollow surface of the cathode 19 to increase resulting in greater dissipation of heat at the surface of cathode 19. This, in turn, further increases the evaporation of arsenic from the surface of the cathode 19. This can result in a runaway condition and eventual destruction of the device. With the addition of the electrical conductive sleeve 17, as a cathode fall increases at the face of the cathode 19, the portion of the current ls passing through the sleeve 17 is increased, thereby minimizing the increase in cathode heating. This can be seen by reference to FIG. 5. Curve 50 represents a typical hollow cathode device with materials other than arsenic, selenium, tellurium, antimony, sulfur and phosphorous. When one attempts to use arsenic, selenium, etc. then curve 52 is representative. When one utilizes the structure described herein, curve 54 represents the device.

The device is found to operate most satisfactorily if the cathode fall of the cathode 19 is relatively higher than that of the sleeve 17. I-Iowever, the device will operate when the cathode fall of the cathode 19 is the same as that of the sleeve or if the cathode fall is less than that of the sleeve. The most desirable situation is when the extension of the sleeve 17 forward of the front face 20 of the cathode 19 is greater than the diameter of the bore 21. The length of the extension of the sleeve 17 is at least equal to and may be as great as 20 times the diameter of the bore 21. The diameter of the sleeve 17 should be greater than the diameter of the bore 21 and this diameter should be at least 1.8 times as great as the diameter of the bore 21 and up to 20 times as great.

Any fluctuation in the power supply 31 will have its effect on the hollow cathode reduced by the ratio of It is also found that arcing will usually occur when the cathode is made to rub against the insulator sleeve because of mechanical shock or vibration. The sleeve 17 or conductive coating 24 will effectively separate the cathode 19 from direct contact within the insulator 38 and thereby eliminate the possibility of this arcing effect which produces spurious emission from the device.

In FIG. 4 the effects of the sleeve extension is illustrated. Curve 51 of FIG. 4 illustrates a device without a sleeve extension and in which the extension is substantially zero` Curve 53 illustrates a device in which the sleeve 17 extension is three-sixteenths of an inch and curve 55 illustrates a sleeve with an extension of threeeighths of an inch. lt can be seen from these curves that by operating in the range of about 2O milliamps of current that a relatively uniform spectral output may be obtained even when there may be some fluctuation in the current supplied to the hollow cathode device. It has been found that by utilization of a sleeve 17 of a suitable length that fluctuation in intensity is less than 2 percent. Without the sleeve the fluctuation was found to vary from 3 to 8 percent and much greater once a runaway condition previously mentioned is reached. As previously mentioned, it also prevents excessive current which may destroy the device.

What is claimed is:

1. A hollow cathode device for use as a radiation source lamp comprising an envelope and having an output window, said envelope having a gas fill of a pressure from l to 30 mm. of mercury, a cathode assembly and an anode disposed within said envelope, said cathode assembly comprising a cathode body member comprising a material which emits radiation having defined spectral lines characteristic of the material in response to a discharge between said anode and said cathode body member, said cathode body member having a centrally located bore in the front surface thereof facing said output window, an electrical lead-in electrically connected to said cathode body member, an electrically conductive sleeve member of different material than said cathode body member about said cathode body member and extending beyond said front surface of said cathode body member, the inner diameter of said sleeve member being greater than the diameter of said bore by at least 1.8 times, the length of said extension portion of said sleeve being at least as great as the diameter of said bore, said sleeve member being electrically connected to said cathode body member leadin to provide a discharge between said anode and said sleeve in parallel with discharge between said cathode and anode and electrical shielding means to prevent electrical discharge from said anode to the external surface of said sleeve member.

2. The device set forth in claim 1 in which said cathode body member is comprised of a material of an electrically conductive material and a material selected from the group consisting of arsenic, selenium, tellurium, antimony, sulfur and phosphorous.

3. The, device set forth in claim l in which said cathode body member is an alloy comprised of an electrically conductive material and a material selected from the group consisting of antimony, arsenic, tellurium, and selenium.

4. The device as set forth in claim 1 in which means is also provided for substantially confining the electrical discharge between said cathode and said anode to substantially the bore of said body member. 

1. A hollow cathode device for use as a radiation source lamp comprising an envelope and having an output window, said envelope having a gas fill of a pressure from 1 to 30 mm. of mercury, a cathode assembly and an anode disposed within said envelope, said cathode assembly comprising a cathode body member comprising a material which emits radiation having defined spectral lines characteristic of the material in response to a discharge between said anode and said cathode body member, said cathode body member having a centrally located bore in the front surface thereof facing said output window, an electrical lead-in electrically connected to said cathode body member, an electrically conductive sleeve member of different material than said cathode body member about said cathode body member and extending beyond said front surface of said cathode body member, the inner diameter of said sleeve member being greater than the diameter of said bore by at least 1.8 times, the length of said extension portion of said sleeve being at least as great as the diameter of said bore, said sleeve member being electrically connected to said cathode body member lead-in to provide a discharge between said anode and said sleeve in parallel with discharge between said cathode and anode and electrical shielding means to prevent electrical discharge from said anode to the external surface of said sleeve member.
 2. The device set forth in claim 1 in which said cathode body member is comprised of a material of an electrically conductive material and a material selected from the group consisting of arsenic, selenium, tellurium, antimony, sulfur and phosphorous.
 3. The device set forth in claim 1 in which said cathode body member is an alloy comprised of an electrically conductive material and a material selected from the group consisting of antimony, arsenic, tellurium, and selenium.
 4. The device as set forth in claim 1 in which means is also provided for substantially confining the electrical discharge between said cathode and said anode to substantially the bore of said body member. 